This section provides background information related to the present disclosure which is not necessarily prior art.
Conventional dual-clutch transmissions (DCT) combine two manual transmissions into a single transmission assembly. Accordingly, dual-clutch transmissions provide a design alternative to conventional manual and automatic transmissions and can offer increased efficiency when properly configured for a particular vehicle. With increasing fuel costs, vehicle fuel economy has become an important design consideration in recent years contributing to a rise in the number of dual-clutch transmissions being installed in vehicles, particularly in the automotive market.
Dual-clutch transmissions typically include two layshafts, each supporting a plurality of gears. Each layshaft also has a clutch interconnecting the layshaft to a hub and each clutch connects and disconnects each respective layshaft from the engine. One of the layshafts may include only odd numbered gears while the other layshaft may include only even numbered gears so that clutch-to-clutch power-on shifts can be accomplished in a similar manner to the shifting of a planetary automatic transmission. Accordingly, one of the layshafts may carry a first gear while the other layshaft carries a second gear and so on. The number of forward gear ratios provided thus equals a sum of the number of gears disposed along the two layshafts. Additional gear ratios thus require the addition of odd and even numbered gears to the layshafts, which contributes to a larger, heavier, costlier, and less efficient dual-clutch transmission.
During vehicle launch, conventional dual-clutch transmissions engage the first gear and the clutch that is connected to the layshaft supporting the odd numbered gears. Accordingly, all of the torque from the engine is directed through this one clutch. To reduce the noise, vibration, and harshness of the vehicle launch, some slip of this clutch is permitted until a minimum required vehicle speed is achieved and clutch lock-up torque disturbance is within pre-designated limits. This clutch slip, particularly for extended durations of time, requires an effective cooling strategy for the clutch. Where the clutch is a wet clutch, cooling is achieved by high volume fluid flow through the clutch, which may reach up to 20 liters per minute. Such coolant flow requirements lead to an increase in pump size and/or number, which increases parasitic losses and thereby decreases the efficiency of the dual-clutch transmission. Accordingly, dry clutches are typically more efficient since pump related losses can be reduced or eliminated. However, such dry clutches rely on less efficient air-cooling and repeated launches can lead to overheating of the clutch. Limiting slip time or alternating first gear launches with second gear launches (thus utilizing the clutch for the even numbered gears while the clutch for the odd numbered gears cools) has been used as a strategy to avoid structural damage to the clutch and/or transmission, but there are several drawbacks associated with these strategies. Mainly, torque jerks, slower acceleration, and increased noise, vibration, and harshness occur.